Significant oil and gas resources are at present stored in smaller fields that are not economically feasible to produce. Such fields, in general, termed marginal fields, can give at typical potential production of 10,000 to 40,000 barrels of oil equivalents per day. For some marginal fields the well pressure is below 100 barg, which makes production with existing equipment non-profitable. Further, there is an increasing demand to be able to produce oil and gas in a more environmentally friendly way, which means both reduced emissions to the surroundings and lower demand for power and equipment for the production. Several marginal fields, in particular from depleted reservoirs and sources of high water cut, will by production cause significant danger of hydrate formation, which increases the costs for production significantly.
In patent publication U.S. Pat. No. 6,339,996 B1 a vessel is described, having storage tanks of composite for compressed natural gas. Weight saving for composite tanks, relative to comparable steel tanks, is up to 70%. The tanks according to said patent publication are vertically aligned, and out of consideration for stability a comprehensive pipe system to distribute the load is required. The loading and unloading procedure results in pressure relief of gas that could be found in the tanks or be formed in the tanks, and subsequent recompressing of the gas.
In patent publications U.S. Pat. No. 6,230,809 B1 and U.S. Pat. No. 6,019,174 a vessel is described that can load unprocessed well stream directly into storage tanks. The vessels are without equipment for processing, but it is mentioned that the lighter fractions can be used on board the vessel for energy production. In connection with loading nothing is described about simultaneous unloading of high-pressure gas that could be found in the storage tanks beforehand.
In patent publication U.S. Pat. No. 4,446,804 a method is described for transport of oil and gas under heavy pressure in tanks onboard a vessel. Water under pressure or some other feasible liquid, in a tank or group of tanks to be loaded, is displaced by the load to a new tank or group of tanks. Filling and emptying thereby take place under pressure by use of displacement, so that the pressure relief of the load is avoided. Thereby, significant pressure loss over the control valve in the tank is avoided, which pressure loss typical can be about 100 bar. Thereby evaporation of the load and poor tank utilization, severe cooling and formation of ice and hydrates, in addition to flow velocities up to the speed of sound, with resulting stresses of the tank assembly, are avoided. Nothing is mentioned about having high-pressure load in the tanks beforehand, for unloading and injection thereof together with optional fractions separated out from the load, simultaneously with loading.
In patent publication U.S. Pat. No. 3,830,180, vertically standing tanks for cryogenic fluids are described, arranged in a vessel, around which tanks insulation is provided with a convection barrier. Cooling of the hull is thereby avoided.
In patent publication NO 320013 a system for production of unstable multiphase fluid from a subsea source is described, with simultaneous injection of water and CO2 to the subsea source for pressure support. More specifically, the system comprises:
A seabed-located sub-sea installation having                at least one well for receival of well stream,        at least one water separator for separation of water from the well stream,        at least one injection pump for injection of the separated water through        at least one water injection well,        
at least one riser for transport of multiphase fluid to
at least one loading/unloading station where a vessel can lay anchored and load/unload
connection for power and signals from the vessel to the sub-sea installation
at least one riser for transport of CO2 from a vessel to
at least one wellhead for injection of CO2 delivered from
at least one vessel which at the above-mentioned station can unload CO2 and simultaneously load multiphase fluid for transport to
a receival plant that can receive unstable multiphase fluid for utilization thereof, and from which CO2 can be delivered for transfer to the vessel for transport and subsequent injection into the underground source.
For continuous operation of the system according to NO 320013 preferably two vessels, of which each vessel preferably has tanks coupled in series, are used, in such a way that when multiphase fluid is loaded, CO2 will be displaced and thereby unloaded, optionally by means of a compressor in order to ensure sufficient pressure for injection into the underground source. At the interface between load and CO2 preferably a plug following the fluid flow is used, and plug sluices are arranged at the end of the serial connection. The tanks of the vessel in the above-mentioned system are constructed of coiled tubing of reinforced polymer material. In the above publication there is no description of loading and unloading by displacement in both ends of the load travel of the vessel, no description of storage tanks on vessels so that natural separation takes place in the tanks, are described, the system is not applicable to marginal fields with low well pressure, specific measures against hydrate formation are not described, and situations exist when significant burning of gas will have to take place because of safety considerations.
There is a demand for a system, a method and vessel for production of oil and heavier gas fractions that are advantageous in relation to the above-mentioned problems.